Mesh arms twisted when connected to armature
Your arms IKs have their pole targets set to your legs pole targets.
You forgot to set the pole angle to 180° on your right arm as you did on the left side.
Your Hand.R bone is rotated in pose mode. Go in pose mode and hit &#9095; AltR to reset its rotation.


For your IK chain to fold the right way, there are two things you can do:


Already fold the chain the way it's meant to fold in edit mode (just a little), so the IK solver knows in which direction to work;

If it's not enough, select the bone with the IK constraint in Pose mode, head over to the Bone tab, Inverse Kinematics panel, and lock the local axes you don't want that IK chain to fold on. Here for your left arm it's the Z axis you want to fold on, so lock the others. That's also where you can indicate minimum/maximum angles so your character don't look like he brokes his bones all the time.